The invention relates to a method and a device for evaluating defects in flat textile structures.
Hitherto, the procedure for the inspection of fabrics has been, for example, that training of the inspecting personnel in admissible and inadmissible defects in the fabric was based on examples. In the actual inspection, it was then sought to detect defects visually and to assign them to an already known example. Defects which appeared to correspond to examples which were unacceptable were marked as such and others were subsequently ignored, as acceptable defects.
In automatic inspection systems, images of the fabric were subjected to filters which filtered out characteristics of the image, and these were then compared with predefined thresholds. Depending on the result of the comparison, possible defects in the images were marked or simply ignored.
A disadvantage of the first method is the fact that human evaluation of a defect is subjective, and is not always uniformly reproducible. Moreover, the person experiences doubt when there are borderline cases. The latter are then not always treated in the same way. This type of inspection thus results in a non-uniform evaluation of fabric defects. Moreover, this type of evaluation does not always proceed at the same rate. In particular, it is slowed down if the defect at issue is difficult to classify and doubts delay the decision.
In the case of the second method, it is a disadvantage that the above-mentioned comparison with threshold values relate to parameters which do not relate directly to the image or the defect. For example, such a parameter can be a limit frequency which, although it does have a significance for the image, acts in combination with other parameters and, considered in isolation, has an effect on the image which is identifiable only with difficulty and is not always uniform. Such methods have been based particularly on what can be easily automated, but it has had to be accepted that the result is not always definite and does not always serve the desired purpose.
It is evident that speck-type, punctiform or linear defects of comparatively small extent in flat textile structures are perceived in relation to the structure of the surface of such a flat textile structure. Thus, in very homogeneous surfaces, extremely small irregularities are perceived clearly by the eye and appear as disturbing defects. On the other hand, an irregular surface pattern reduces the conspicuousness of small imperfections. A problem consists in finding a valid quantity for the characteristic of the defects which conforms to subjective perception. In particular, an interpretable standard is required for setting the sensitivity on automatic inspection systems.
The object of the invention, as characterized in the claims, is to create a method and a device which permit the repeated, rapid and unambiguous evaluation of defects in flat textile structures.
This is achieved according to the invention in that, according to the method, an image of a flat structure is generated such that at least two representations of defects in the flat structure are generated which differ in respect of the length and contrast of the defect. On the basis of these representations, a decision is made as to whether a defect in the flat structure is admissible or inadmissible. Particularly advantageous is a tabular or matrix-like arrangement of representations of different kinds of defects, an image of the defect-free flat structure being used as background. The representations of the defects can thus be evaluated in the surrounding which is specific to the task. To this end the image is divided into sections by means of a two-dimensional grid, the length of the defects constituting the first dimension and the contrast of the defects constituting the second dimension. Sensitivity curves are also superimposed on the image.
The advantages achieved by the invention lie particularly in the fact that a method is achieved by which defects in flat textile structures can be evaluated both objectively and by automated means. The method is also suitable for setting the sensitivity of automated inspection systems for flat textile structures. The invention also provides for a device by which automatically and objectively inadmissible defects in flat structures can be separated out without direct human evaluation. According to the invention, it is possible to evaluate defects in a flat structure which result from both the structure and the material used. In the case of a fabric this means that it is possible to evaluate both defects in the alignment of individual weft or warp threads and defects caused by the yarn itself such as, for example, diameter variations.